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[New] Implemented CRC CCITT in Crc.crcCcitt() using a lookup table

This commit is contained in:
Robert von Burg 2018-03-26 12:09:04 +02:00
parent e8783e9749
commit cd06e7b5db
1 changed files with 24 additions and 14 deletions
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38
li.strolch.utils/src/main/java/li/strolch/utils/Crc.java
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@ -3,10 +3,30 @@ package li.strolch.utils;
import static li.strolch.utils.helper.ByteHelper.toByteArrayBigEndian;
/**
* Implements Cyclic Redundancy Checks
* <p>Implements Cyclic Redundancy Checks</p>
*
* <p>Fast byte-wise CRC16 calculation, using the code from Christian d'Heureuse, Inventec Informatik AG, Switzerland, www.source-code.biz</p>
*/
public class Crc {
private static final short[] crcTable;
static {
int poly = 0x1021;
crcTable = new short[256];
for (int x = 0; x < 256; x++) {
int w = x << 8;
for (int i = 0; i < 8; i++) {
if ((w & 0x8000) != 0) {
w = (w << 1) ^ poly;
} else {
w = w << 1;
}
}
crcTable[x] = (short) w;
}
}
/**
* Implements a CRC-CCITT (XModem), using the initial value 0x0000 and the polynomial 0x1021
*
@ -32,20 +52,10 @@ public class Crc {
* @see <a href="https://www.lammertbies.nl/comm/info/crc-calculation.html">crc-calculation</a>
*/
public static short crcCcittBytes(byte[] bytes) {
int crc = 0x0000; // initial value
int polynomial = 0x1021; // 0001 0000 0010 0001 (0, 5, 12)
for (byte b : bytes) {
for (int i = 0; i < 8; i++) {
boolean bit = ((b >> (7 - i) & 1) == 1);
boolean c15 = ((crc >> 15 & 1) == 1);
crc <<= 1;
if (c15 ^ bit)
crc ^= polynomial;
}
int crc = 0x0000;
for (byte aData : bytes) {
crc = ((crc << 8) & 0xFF00) ^ (crcTable[(crc >> 8) ^ (aData & 0xFF)] & 0xFFFF);
}
crc &= 0xffff;
return (short) crc;
}
}